A variety of plastic composite structures have been proposed in the past in which a polar, oxygen barrier resin is adhered to a modified polyolefin resin, frequently a polyethylene resin, which has been chemically modified by grafting varying amounts of an unsaturated carboxylic acid or an unsaturated carboxylic acid anhydride, frequently a dicarboxylic acid anhydride, at various levels to the polyethylene backbone by methods known in the art. In some cases a polyethylene is bonded to a polar oxygen barrier resin with an adhesive which is a modified polyolefin containing various levels of grafted carboxylic acid or dicarboxylic acid anhydride and usually also an amorphous olefin rubber, such as ethylene-propylene diene rubbers, ethylene propylene copolymers, or linear low density polyethylenes which provide toughening and improve adhesion to polar substrates. Generally it has not been possible to obtain both good adhesion of the polyolefin resin to the polar resin and high moisture vapor barrier properties in the polyolefin resin. It is known that high density polyethylene which has a high crystallinity provides better moisture barrier properties than low density polyethylene or linear low density polyethylene or ethylene/propylene rubbers but there have been problems in obtaining adequate adhesion of high density polyethylene or modified high density polyethylene to polar oxygen barrier resins. Frequently the unsaturated carboxylic anhydride employed has been chi-methylbicyclo(2.2.1)hept-5-ene-2,3-dicarboxylic acid anhydride or maleic anhydride, but many other anhydrides have been disclosed in other patents. Illustrative of these types of composite structures known in the art are shown in U.S. Pat. No. 4,087,587, Shida et al., U.S. Pat. No. 4,198,327, Matsumoto et al., U.S. Pat. No. 4,230,830, Tanny et al., U.S. Pat. No. 4,409,364, Schmukler et al., U.S. Pat. No. 4,460,646, Inoue et al., U.S. Pat. No. 4,487,885, Adur et al., and U.S. Pat. No. 4,510,286, Liu. Most commonly, these patents disclose composite structures involving carboxylic acid grafted ethylene polymers or a carboxylic acid grafted polypropylene adhered to crystalline polycarboxylamide such as nylon-6. In some cases, adhesion to superior oxygen barrier materials such as EVOH (ethylene vinyl alcohol copolymer) are described.
U.S. Pat. No. 4,416,944, Adur describes composite structures of modified polyethylene and polypropylene adhered to oxygen barriers such as EVOH or nylon and also shows adhesion to high density polyethylene of modified polyolefin compositions comprising high density polyethylene having a density in the range of 0.94-0.97 g/cc, high density polyethylene grafted to chi-methylbicyclo(2.2.1)hept-5-ene-2,3-dicarboxylic acid anhydride at a level of 1.5 weight percent together with a polypropylene resin and a linear low density polyethylene resin having a density in the range of 0.91-0.94 g/cc. The total amount of high density polyethylene in the adhesive composition is stated to be in the range of 20-60% by weight in the examples.
U.S. Pat. No. 4,481,262, Shida et al., describes composite structures adhered to nylon-6 or ethylene vinyl alcohol copolymer in which the hydrocarbon copolymer adhered to it is a composition containing a linear low density polyethylene having carboxylic anhydride grafted to it and blended with a variety of different materials including ethylene vinyl acetate copolymer, ethylene methyl acrylate copolymer, low density polyethylene homopolymers or linear low density copolymers. In the adhesive compositions, the grafted linear low density polyethylene comprises 10% and the other materials 90% of the blend. In the examples ethylene vinyl acetate copolymer or an ethylene acrylate copolymer or an ethylene methyl acrylate copolymer are illustrated as comprising 90% of the adhesive blend but in one example a polyethylene having a density of 0.94 g/cc was substituted for these copolymers and provided some adhesion to nylon.
U.S. Pat. No. 4,460,632, Adur et al. discloses composite structures in which an adhesive polyethylene blend is adhered to substrates such as nylon, nylon-6, polyethylene or ethylene/vinyl alcohol copolymer. The adhesive compositions disclosed are blends of a medium density high pressure, free-radical polyethylene, a linear low density polyethylene and a high density polyethylene graft to a carboxylic anhydride such as chi-methylbicyclo(2.2.1)hept-3-ene-2,3-dicarboxylic anhydride or in one example maleic anhydride. In the adhesive compositions employed in these composites, linear low density polyethylene comprises from 10-90% by weight according to the generic disclosure. The examples show 10% of the grafted high density polyethylene and a total of from 10-90% of the mixture of grafted high density polyethylene and medium density polyethylene. It is shown that the grafted high density polyethylene may contain a very wide range of acid anhydride grafted to it generically stated as from 0.05-30 weight percent.
Another patent of interest is Mito et al., U.S. Pat. No. 4,370,388. This patent discloses adhesive compositions and composite structures made from them. Broadly adhesive structures are disclosed which contain from 97-50 parts by weight of a polyethylene resin having a density in the range of 0.945-0.970 grafted with a dicarboxylic acid anhydride such as maleic anhydride, 3-50 parts by weight of an ethylene/4-methyl-1-pentene copolymer having an ethylene content of 93-99.9 mole percent and 0-20 parts by weight of a rubbery synthetic polymer or copolymer. The patent broadly discloses that the amount of the grafted monomer in the grafted high density polyethylene may range from 0.001-10%, more preferably 0.02-5%. Comparative examples in which the copolymer of ethylene/4-methyl-1-pentene copolymer was replaced with an ethylene/hexene-1 copolymer or an ethylene/propylene copolymer rubber are said to be unsatisfactory. The density of the ethylene/4-methyl-1-pentene copolymer disclosed is from 0.910-0.945 g/cm.sup.3 or preferably 0.920-0.93 g/cm.sup.3. Primarily two layer composites are contemplated in which, in addition to the adhesive resin, nylon-6, nylon-66 and other similar crystalline nylons as well as a variety of polyesters and saponified copolymers of ethylene/vinyl acetate are contemplated. The only exemplified grafted high density polyethylene employed is one containing 2% by weight of maleic anhydride, a melt index of 7 g/10 min and a density of 0.962 g/cm.sup.3.
In all of the above patents the peel strengths disclosed are substantially impossible to relate to each other because they are so dependent upon laminating conditions and unstated percentages of carboxylic anhydride grafted to a polyolefin.
Generally peel strength for coextruded composites are not shown.